Performance regression from marking `c_PQnfields` safe

[ChickenProp]

I had a benchmark slow down significantly when upgrading from 0.9.5.0 to 0.10.0.0. On profiling, the problem was that the ~2.3 million calls to nfields went from taking almost no time to taking ~1s.

c_PQnfields was marked safe in #48 because it's hard to know whether libpq functions are actually okay to be unsafe or not. In this case, looking at https://github.com/postgres/postgres/blame/50e6eb731d98ab6d0e625a0b87fb327b172bbebd/src/interfaces/libpq/fe-exec.c#L3489, the code is:

int
PQnfields(const PGresult *res)
{
	if (!res)
		return 0;
	return res->numAttributes;
}

I'm pretty sure that would be safe as-is; and it seems it hasn't been updated since 1999. (Unless lines have been added and then removed.)

So I wonder if this specific function could be marked unsafe again?